It is known to measure the combustion processes taking place in the combustion chamber of an internal combustion engine from various standpoints by ascertaining various physical parameters. Among these, for instance, is ascertaining the course of the combustion process over time and through space; ascertaining the instant fuel of injection; ascertaining fuel injection processes; and measuring irregular combustion processes.
Among these irregular combustion processes is "knocking", which occurs in internal combustion engines under certain operating conditions. Knocking, as usually understood, is caused by oscillations within the audible frequency band of the compressed fuel-air mixture, which are triggered by a shock wave. During such oscillations, the transmission of heat to the piston walls and cylinder walls of the engine is substantially increased. A thermal overload of these surfaces is the result, so that knocking must be avoided. On the other hand, it is desirable to utilize the available working range of the engine to the fullest possible extent; it is thus necessary to provide means which will reliably indicate incipient knocking, so that it is then possible to realize a means of engine control in which the engine is always operated just below the knocking limit.
In addition to the problem of furnishing transducers suitable for ascertaining knocking, there is also the measurement problem of being able to recognize the knocking signal reliably and without interference on the basis of the engine oscillations ascertained by the transducer, so that the engine can be appropriately regulated in accordance with a "yes" or "no" signal for knocking recognition.
A knocking detector for this purpose is described in U.S. Pat. No. 3,540,262, where the measured knocking signal is compared with a predetermined threshold signal which is independent of the engine signal; if the threshold is exceeded, a knocking recognition signal is emitted. This apparatus has the disadvantage, however, that only a comparison with an external signal independent of the engine is performed; there is no recognition of knocking with respect to background engine noises.
U.S. Pat. No. 4,012,042 furthermore describes a knocking detector in which the measured knocking signal is compared with a reference signal, which is generated via a function generator in accordance with engine speed. This procedure has the disadvantage, however, that again the actual background noise of the engine is not taken into consideration, but instead is only simulated via a function generator. In particular, no attention is paid to the mode of operation at the time, the engine setting, or the aging of the engine.
Finally, a method for ascertaining engine knocking is known from German Patent Disclosure Document No. 29 16 591 to which U.S. Pat. No. 4,271,469 Kawaii et al, corresponds, in which the peak value of the knocking signal is monitored and compared with the peak values preceding it which have been ascertained for preceding combustion cycles. Among other provisions, measurement is performed only during the existence of a measuring or strobing window, that is, within an interval of time which has been correlated with a specific angular range for crankshaft rotation.
However, the apparatus and methods discussed above have the disadvantage that disturbance noises not related to knocking are not always filtered out reliably, so that measurement results are adulterated. This applies particularly to the noise signals occurring during so-called piston tipping, the amplitude of which can cause a substantial overlap with the knocking signals.